No, it can be a gotcha because the json column type (which still exists and many may mistakenly use instead of the jsonb type) allows it:

select '{"foo": 1, "foo": 2}'::json; json ---------------------- {"foo": 1, "foo": 2} (1 row)

select '{"foo": 1, "foo": 2}'::jsonb; jsonb ------------ {"foo": 2} (1 row)

So technically, the gotcha is you have to remember to use jsonb instead of json, as well as json having a true "gotcha" and jsonb not having one.